A phrase used to indicate that a romantic or sexual relationship has ended, or is, as when the wheels do fall off a wagon, kaput. Sometimes heard as "wheels fall off the wagon" or "wheels falling off the wagon"

"She found out he was also shagging her brother and so the wheels fell off the wagon then."

"Her incessant screwing around on him made the wheels fall of the wagon, and he dumped her."

"If he keeps that up, I can see the wheels falling of the wagon soon."